**Core Concept**
Restoration of a tooth planned to be an abutment for a removable partial denture (RPD) should provide sufficient support, stability, and retention to withstand masticatory forces and ensure proper denture function. This requires a restoration that distributes forces evenly, has sufficient surface area for denture retention, and does not compromise the periodontal support of the tooth.

**Why the Correct Answer is Right**
The correct answer is **C. Cast metal restoration**. A cast metal restoration, such as a cast gold or titanium alloy crown, provides excellent support and stability to the tooth, allowing it to withstand the forces exerted by the RPD. The metal material distributes forces evenly, and the crown's surface area provides adequate retention for the denture. Additionally, cast metal restorations do not compromise the periodontal support of the tooth, as they do not cover the entire tooth surface and do not interfere with the periodontal ligament.

**Why Each Wrong Option is Incorrect**
**Option A:** A porcelain-fused-to-metal (PFM) crown may not provide sufficient support and stability for an RPD abutment, as the porcelain may fracture under heavy forces.
**Option B:** An all-ceramic crown may not provide adequate support and stability, as it can be prone to fracture and does not distribute forces as effectively as a cast metal restoration.
**Option D:** A composite resin restoration is not suitable for an RPD abutment, as it may not provide sufficient support and stability, and can be prone to wear and fracture.

**Clinical Pearl / High-Yield Fact**
When selecting a restoration for a tooth planned to be an RPD abutment, consider the forces that will be exerted on the tooth and the type of restoration that will provide the best support, stability, and retention.

**Correct Answer:** C. Cast metal restoration.